1. Field of the Invention
The invention concerns a detector circuit which includes a radiation-sensitive sensor and downstream-connected filter and simplifier stages for outputting a useful signal.
2. Description of the Prior Art
A detector circuit of that kind is known from DE 24 56 162 A1 in the form of a band pass filter which is operated by way of a pre-amplifier from an optronic detector element which is disposed as an optronic sensor in a radiation-sensitive voltage divider.
In accordance with EP 0 315 855 A1 a respective amplifier is actuated from the central tapping of a radiation-sensitive voltage divider so that, when a plurality of detector elements are provided, the number of passage means for passing through a housing can be reduced by single-pole interconnection within the housing. In that arrangement the detector elements involve sensors which are responsive to thermal radiation.
The detector circuit of the general kind set forth is intended to make available a useful signal for further processing in an evaluation circuit, which preferably involves a firing triggering circuit in accordance with German patent specification No 34 109 42 or also German patent specification No 32 102 07, with inter alia a radiation-sensitive voltage divider.
Functioning of the radiation-sensitive voltage divider is based on the consideration that the steady signal level occurring at the central tapping fluctuates in dependence on the irradiation of the detector element and generally falls by virtue of a greater degree of conductivity in relation to more intensive irradiation. This excitation-dependent drop in level which is superimposed on a steady or dc voltage level is identified here as the detector signal which is converted by means of the detector circuit into the useful signal to be outputted.
A particular disadvantage with the detector circuit of the general kind set forth is that the filter stage with its high-pass characteristic for separation of the fluctuating detector signal from the steady signal level involves the occurrence of capacitive charge reversal phenomena which are troublesome, as they last for a long time, in particular when the aim is to achieve a high pass edge or corner frequency which is as low as possible, as for example when using that detector circuit in a seeking fuse sensor for target acquisition purposes. If therefore for example strong but only momentary excitation of the detector element is implemented by the received radiation (as in the case of a flash of light in relation to an optronic detector element or when the situation involves pivoting over a locally limited heat from a fire with a thermal detector element), then that, in the series capacitance of the high pass filter, results in the displacement of a very large quantity of charge. That potential displacement must be reversed again as quickly as possible when the extreme radiation excitation is terminated so that the detector circuit again then furnishes a useful signal which follows the normal intensity of radiation sources which are really of interest. The high charge reversal time constant as a result of low high pass edge frequency however means that strong charging of the series capacitance only reverts with a delay to the rate of the reduced excitation; while a reduced excitation following the strong excitation, because of the high charge reversal time constant of the series capacitance and overdriving of long duration, resulting therefrom, of the following signal amplifier is initially not evaluated at all until the charging of the coupling capacitor has fallen again with the long time constant to the potential of the sensor-governed potential fluctuations.
There is therefore also the disadvantage that the signal amplifier which is connected on the output side of the high pass filter is immediately overdriven by a high displacement current and is then initially still held in the overdriven mode until the charge reversal phenomena have sufficiently died away again in accordance with the given time constant. As a result the signal amplifier only returns to its linear working range again, for the output of a useful signal which can be utilised, when the extreme detector excitation has long ago decayed; with the consequence that, during a certain period of time, even after decay of the extreme excitation, the normal ambient factors which are detected by sensor means still cannot be processed again. That problem becomes all the more serious in a practical context as the steady signal level, which is relatively high due to the equalisation action involved, at the central tapping of the radiation-sensitive voltage divider does not allow a high level of pre-amplification upstream of the high pass device. because otherwise synchronisation errors in sensors operating in parallel would be excessively amplified; while on the other hand pre-amplification would be something to strive for, in the interests of an improvement in the signal-noise ratio in the useful signal.